1. Field of the Invention
The present invention relates to a gas separation composite membrane, a gas separation module, a gas separation device, a gas separation method, and a method of producing a gas separation composite membrane.
2. Description of the Related Art
A material formed of a polymer compound has a gas permeability specific to the material. Based on this property, it is possible to cause selective permeation and separation out of a target gas component using a membrane formed of a specific polymer compound. As an industrial application for this gas separation membrane related to the problem of global warming, separation and recovery from large-scale carbon dioxide sources using this gas separation membrane has been examined in thermal power plants, cement plants, or ironworks blast furnaces. Further, this membrane separation technique has been attracting attention as a means for solving environmental issues which can be performed with relatively little energy. In addition, natural gas or biogas (gas generated due to fermentation or anaerobic digestion, for example, biological excrement, organic fertilizers, biodegradable substances, sewage, garbage, or energy crops) is a mixed gas mainly containing methane and carbon dioxide, and a membrane separation method is being examined as a means for removing impurity components such as the carbon dioxide and the like (JP2007-297605A).
When natural gas is purified using a membrane separation method, excellent gas permeability and separation selectivity are required in order to efficiently perform separation of gas. Various membrane materials have been examined in order to realize excellent gas permeability and separation selectivity, and a gas separation membrane using a polyimide compound has been examined as a specific example thereof.
Moreover, in an actual plant, a membrane is plasticized due to high pressure conditions and impurity components (for example, benzene, toluene, and xylene) present in natural gas and this leads to degradation of separation selectivity, which is problematic. In order to prevent plasticizing of the membrane, it is known that introduction of a cross-linked structure or a branched structure to a polyimide compound constituting the membrane is effective (for example, JP2013-188742A, JP2013-169485A, JP2013-046904A, JP2013-046903A, JP2013-046902A, and JP2013-027819A).
In order to obtain a practical gas separation membrane, it is necessary to ensure not only gas separation selectivity but also sufficient gas permeability by making a gas separation layer thinner. As a method for this, a method of making a portion contributing to separation into a thin layer referred to as a dense layer or a skin layer by forming a polymer compound such as a polyimide compound into an asymmetric membrane using a phase separation method may be exemplified. In this asymmetric membrane, a dense layer is used as a gas separation layer and a portion other than a dense layer is allowed to function as a support layer responsible for the mechanical strength of a membrane.
Further, in addition to an asymmetric membrane, a form of a composite membrane in which a substance responsible for a gas separation function is different from a substance responsible for the mechanical strength is also known. In this composite membrane, a gas separation layer formed of a polymer compound such as a polyimide is formed on a gas permeating support responsible for mechanical strength. As a method of producing such a composite membrane, a method of forming a gas separation layer by coating a gas permeating support with a coating solution prepared by dissolving a polymer compound such as a polyimide to form a film is known. In addition, a method of forming a composite membrane by preparing a dense layer formed of a polyimide compound separately from a gas permeating support and then disposing the dense layer on the gas permeating support as a gas separation layer is also known. For example, JP1995-236822A (JP-H07-236822A) describes a method of forming a composite membrane by forming a dense thin film of a polyimide compound on a glass plate, immersing this glass plate in pure water, peeling the thin film from the glass plate in water such that it floats on the surface of the water, scooping the thin film onto a gas permeating support, and drying the film.
It is known that a gas separation layer using a polyimide compound is insolubilized because a cross-linked structure is formed when it is subjected to a heat treatment at a high temperature and the gas separation performance thereof is thus improved (for example, WO2009/091062A). Further, a gas separation layer formed to include an insoluble polyimide compound is formed using a solution of polyamic acid, which is a precursor of a polyimide compound, or the like and can be prepared by this being subjected to a heat treatment at a high temperature for imidization.